Some work machines including construction machines such as hydraulic excavators are equipped with a hydraulic pump driven by an engine and a plurality of hydraulic actuators driven by hydraulic pressure from the hydraulic pump. When two hydraulic actuators of this type of hydraulic work machine are operated simultaneously via separate operating devices to carry out a predetermined operation (combined motion), it is important to match the operating speeds of the two hydraulic actuators so as to provide the operator with a good operational feeling.
For example, suppose that a hydraulic excavator furnished with a hydraulic cylinder for rotating the boom (boom cylinder) and a hydraulic motor for swinging the upper swing structure performs a boom raising operation (swing boom raising) while swinging the upper swing structure. In such a case, it is preferred that the speed of the upper swing structure and that of the boom match each other when the control levers of the actuators have been maximally tilted, and the boom have been raised up to a body (a vessel) of a dump truck when the upper swing structure has been swung to the body position of the dump truck. For this reason, this type of hydraulic excavator has the power distribution to the boom cylinder and hydraulic motor adjusted optimally beforehand, for example, by control valves such that the speeds of the two actuators will correspond when a swing boom raising operation is performed.
Apart from the work machines that possess only hydraulic drive systems as described above, development is underway for hybrid work machines that have both a hydraulic drive system for driving hydraulic actuators and an electric drive system for driving electric actuators. Some of such hybrid work machines are exemplified as a hybrid hydraulic excavator equipped with a hydraulic pump driven by an engine, an hydraulic actuator driven by hydraulic pressure from the hydraulic pump, electric equipment that is capable of supplying electricity without affecting the absorption torque of the hydraulic pump (e.g., an electric storage device such as a secondary battery or a capacitor for storing the electric power generated by a motor generator), and an electric actuator driven by power from the electric equipment.
The hybrid work machines, which are capable of driving the hydraulic actuators and electric actuators by independent power sources, cannot adopt the technology of the aforementioned work machines (hydraulic excavators) that possess only the hydraulic drive system. For example, if the hybrid hydraulic excavator has no torque limits set on the swing motor during a combined motion involving the swing motor and boom cylinder, the swing speed of the upper swing structure tends to be higher than the boom raising speed. As a result, the swing speed and the boom raising speed may fail to match each other, leading to lower operability.
In view of the above problem, there is a work machine possessing control means which puts constraints on the torque or operating speed of the electric actuator (WO 2007/052538) when the hydraulic actuator (boom cylinder) and electric actuator (swing motor) are operated in combination. This work machine further distributes its power to the hydraulic and electric actuators such that the maximum output power of the engine or the output ratio between the hydraulic actuator and the electric actuator will approach a predetermined value; the distribution is in a manner that reduces the absorption power of the hydraulic pump in inverse proportion to the power of the electric actuator being raised.